PROJECT SUMMARY Diet beverages sweetened with artificial sweeteners (AS) occupy a unique category in the food environment as they are a source of intensely sweet taste with no calories or nutrients. Diet beverages are the single largest contributor to AS intake in the U.S. diet, and consumption of diet beverages has significantly increased over the past 30 plus years in concert with the twin epidemics of obesity and type 2 diabetes. A burgeoning body of basic, clinical, and population research suggests that diet beverages are linked with obesity, decline in kidney function, and increased risk of metabolic syndrome, type 2 diabetes, and cardiovascular disease. Other developing research has suggested diet beverages and the AS that sweeten them may alter gut hormones, the gut microbiome, taste preferences, satiety, and overall dietary intake, thus providing plausible mechanisms whereby disease risk may be altered. These initial reports, and related hyperbolic media coverage, have sparked both scientific and public interest in the role that diet beverages may play in health. Despite this budding framework of knowledge, the evidence base is largely uninformed by randomized clinical trials testing whether intake of diet beverages impacts measures of clinical risk. Furthermore, an issue central to this topic is the lack of research addressing the population with diabetes, perhaps the most important population with respect to diet beverage intake. People with diabetes are the highest consumers of diet beverages, tending to consume them as a replacement for dietary sources of sugar, especially in place of sugar-sweetened beverages. This behavior has been endorsed by dietetic and scientific organizations, and diet beverages are marketed as being synonymous with better health, suitable for weight loss, and thus advantageous for diabetes control. The underlying public health concern is that there are few data to support or refute the benefit or harm of habitual diet beverage consumption by people with diabetes. To begin addressing this important scientific gap we propose to test the effect of diet beverage intake on diabetes control parameters in free-living adults with type 2 diabetes in a randomized, two arm parallel trial with a run-in period of 2-weeks and an active intervention period of 12-weeks. We will recruit 240 patients with type 2 diabetes who are usual consumers of commercial diet beverages and randomize them to receive and consume either: 1) A commercial diet beverage of choice (3 servings or 24 oz. daily); or 2) Unflavored bottled water of choice (sparkling or plain) (3 servings or 24 oz. daily). The primary outcome will be a central measure of clinical diabetes control in glycated hemoglobin (HbA1c), and we will also measure cardiometabolic risk and kidney function. Additionally, we will measure plausible mechanisms whereby diet beverage intake may alter risk by assessing the effect of diet beverage intake on the functional composition of the gut microbiome as well as usual dietary intake, appetite, and taste preferences.